I think as students, we can't help but be drawn to the A. It's sort of similar to how a dog can't resist a treat, since it's such a great thing to have. That doesn't mean that we need it, though, just like how the dog can still live off of just eating nasty canned dog food. So that "nasty canned dog food" for us is an F, or whatever the grade you consider a fail is by your own personal standards. And while you can be successful in so many different ways that grades seem insignificant when comparing all of them, you also have to take into consideration how many of these ways can not be applied to a single person. For example, many people who have made money off of technology such as Bill Gates and Steve Jobs were only able to do it because they had a genuine interest in technology and were extremely bright. Not all of us have these qualities, and this example can be used in all areas where success was achieved.
So that is why I think that depending on what area you want to succeed in, it will also becoming a limiting factor towards your success. The professions that you are not interested in will be unable to you, and that subtracts from the total number of ways that you can success. I consider the total nombuer to be countless as a whole but very limited when looking at it for an individual simply because you can not be interested in everything. The way this ties into grades is in the discovery of what your interests are, and also the degree to which they actually help. Having good grades allows you to go to better schools and study what you desire to study. If I wanted to become a writer and had good grades in Literature, I may be able to go to a nice school and study this. However, if I have bad grades and don't go to college, I would have to start without the knowledge college would have provided. That doesn't necessarily mean that success is completely impossible, but it does make it much more difficult.
The reason why I think grades is also the discovery of your interests is because you don't know what you want to study and what you are skilled in at first. All students go to primary school without knowing anything about what they want to study. However, by going to high school and choosing your classes for the first time, you can choose these things and decide what kind of profession you would like to go in. If your grades are high for a certain class, you will discover that you are talented at that subject.
All of my observations were only for a being successful in your career. There are so many other ways of being successful, such as being rich and having a happy family. However, I found that those are much more difficult to tie in with grades since they have more moral value, so I only wrote about this single definition of success.